In order to improve fuel economy or purifying level of exhaust gas, there has been a demand in recent years for measuring a humidity of an intake air that is to be drawn into an engine. Thus, such an intake air flow rate measuring device is equipped with a humidity sensing element to measure a humidity of an intake air as well as a flow rate (refer to, for example, JP 2010-151795 A).
JP 2010-151795 A discloses an intake air flow rate measuring device that is inserted into an intake air duct and is equipped with a humidity sensing element.
Typically, a humidity ratio has been used for controlling an engine. The humidity sensing element for detecting such a humidity ratio includes, in addition to a humidity detector to detect a relative humidity, a temperature detector to detect a temperature of the humidity sensing element itself. Then, the humidity sensing element calculates a humidity ratio based on the relative humidity detected and the temperature of the humidity sensing element.
However, as shown in FIG. 19 illustrating curves of a relative humidity with respect to a temperature, the slope of each of the relative humidity curves increases as the temperature increases. Therefore, a detection error may increase as the temperature increases.
In the intake air flow rate measuring device disclosed in JP 2010-151795 A, heat in an engine compartment transfers to the humidity sensing element in the intake air duct through, e.g., a flange of the intake air flow rate device. As a result, the temperature of the humidity sensing element may be increased by the heat.
If the humidity sensing element is heated, the detection error may increase as described above, and therefore accuracy of detecting a humidity may be deteriorated.
With reference to FIG. 19, one example situation where detection accuracy is deteriorated due to an increase of the temperature of the humidity sensing element will be described. In this example, detection accuracy of a relative humidity is set to be within ±3% RH, and a humidity ratio of an intake air is 18.8 g/kg. When the temperature of the intake air is 30° C., the detection error range of the humidity sensing element is 18.3-19.7 g/kg. However, when the temperature of the humidity sensing element increases to 50° C. due to, e.g., heat in the engine compartment, the detection error range of the humidity sensing element is substantially deteriorated to 16.4-21.3 g/Kg.
In view of the above, it is an objective of the present disclosure to provide an intake air flow rate measuring device that is configure to avoid deteriorating a detection error of a humidity due to an increase in a temperature of a humidity sensing element due to heat in an engine compartment.